1. Field of the Invention
The present invention relates to an optical pickup for high-density information writing and reading systems, and more particularly, to an optical pickup capable of reducing chromatic aberration that occurs when a blue light source is employed.
2. Description of the Related Art
In optical writing and reading systems, the recording density is determined by the size of a focused spot. In general, the size of the focused spot (S) is proportional to a wavelength (λ), and inversely proportional to a numerical aperture (NA) as expressed by the formula (1):S∝λ/NA  (1)
For a higher recording density than is achieved with compact disks (CDs) or digital versatile disks (DVDs), the size of the spot being focused on an optical disk must be further reduced. To reduce the spot size, as can be inferred from the formula (1), the wavelength (λ) of the laser beam must be reduced, and the NA of the objective lens must be increased. Thus, for such high density information recording, a laser beam having a short wavelength such as a blue laser, must be employed as a light source, and the NA of the objective lens must be maintained to be 0.6 or more.
FIG. 1 is a graph showing the variation of emission wavelength of a laser diode with respect to output power at various temperatures of the laser diode case. FIG. 2 is a graph showing the variation of the index of refraction with respect to the wavelength for various optical materials. Referring to FIG. 1, as the output power increases at a particular case temperature, the wavelength of emission increases proportionally, which is a feature of laser diodes. As shown in FIG. 2, the index of refraction of various optical materials sharply varies in a short wavelength region, for example, near 400 nm, compared to at 780 nm for compact disks (CDs) and at 650 nm for digital versatile disks (DVDs).
When writing information to an optical disk, a desired position on the optical disk is located using reading power, and then a recording mark is made at the desired position by increasing the output to writing power. However, such a sudden variation in output power causes a chromatic aberration in the optical system, thereby defocusing the optical spot on the optical disk. In addition, it takes considerable time to correct the defocusing by the control of a servo circuit.
Further, when a high frequency (HF) module is used to reduce noise caused by light reflected from an optical disk toward a light source, the wavelength of light emitted from the light source increases, thereby increasing the chromatic aberration in the optical system and, in particular, in the objective lens. This causes a deterioration in the quality of a reproduction signal. Lastly, it should be further considered that, as can be seen from FIG. 1, the wavelength of emission increases with temperature inside of the optical pickup, and the wavelength variation from using different light sources changes the chromatic aberration.
Various optical pickups, which have a light source having a wavelength of 650 nm and an objective lens, have been suggested so as to be compatible with 0.6 mm-thick DVDs and 1.2 mm-thick CDs. Among the techniques used in the suggested optical pickups are an annular shielding technique for blocking light passing through an intermediate area between far axis and near axis areas, a method for controlling the NA of an objective lens by using liquid crystal (LC) shutters, and a hologram optical element (HOE) technique for splitting light using a HOE to form individual focuses onto two disks having different thicknesses. However, for a compact disk recordable (CD-R), the reflectivity with respect to the red light source sharply drops, and thus a light source having a wavelength of 780 nm is necessary. For this reason, the use of a DVD indefinite/CD definite optical system that is compatible with light beams of both 780 nm and 650 nm, or the use of an objective lens having an annular focus region between far axis and near axis regions has been suggested. In particular, for a CD definite optical system, the NA of the objective lens is reduced and the divergent light is incident on the objective lens, thereby correcting the aberration caused by the difference in the thickness of disks and the objective lens.
An optical pickup using a short wavelength light source is required for higher density information writing and reading than DVD systems are capable of. As an example, for an optical pickup for HD-DVDs, laser light having a wavelength shorter than 650 nm used for DVDs, is required as a light source. However, as previously explained with reference to FIG. 2, since the index of refraction of optical material of the disk varies sharply at wavelengths shorter than 650 nm, excessive aberration occurs. Thus, there is a need for an optical system compatible with existing DVDs, and capable of effectively reducing chromatic aberration.
For a DVD-R, the reflectivity with respect to light sources other than the red light source decreases. Thus, for compatibility with DVD-Rs, a light source having a wavelength of 650 nm must also be used. However, the problem of aberration can not be eliminated from a 400 nm-objective lens by only controlling the divergence of the light emitted from the 650 nm-light source and incident on the objective lens. Thus, the critical concern in developing HD-DVD compatible systems is finding an effective chromatic aberration correction technique.
An example of a conventional objective lens capable of correcting chromatic aberration, which was described in Japanese Patent Laid-open Publication No. Hei 10-123410, is shown in FIG. 3. Referring to FIG. 3, the conventional objective lens is constructed of two lenses: a first lens 3 and a second lens 4. In particular, the first lens 3 corrects chromatic aberration and is arranged between a disk 6 and the second lens 4. The second lens 4 focuses the light. This structure allows the NA to be increased to 0.7 or more, so that such an objective lens can be employed in an optical system for high-density recording. However, the objective lens has problems in that use of two lenses increases the optical length of the system, and reproduction of the beam spots is highly sensitive to a change in the relative positions of the two lenses.